https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81196
Bug ID: 81196 Summary: Number of iterations found for p!=q but not for p<q Product: gcc Version: 8.0 Status: UNCONFIRMED Keywords: missed-optimization Severity: normal Priority: P3 Component: tree-optimization Assignee: unassigned at gcc dot gnu.org Reporter: glisse at gcc dot gnu.org Target Milestone: --- void f(short*p){ p=(short*)__builtin_assume_aligned(p,64); short*q=p+256; for(;p!=q;++p,--q){ short t=*p;*p=*q;*q=t; } } compiled with -O3 -march=skylake, this gets vectorized. However, if I replace p!=q with p<q (much safer in case 256 was actually 255), it isn't vectorized anymore, apparently because the number of iterations cannot be determined. Replacing p!=q with p!=q&&(p+1)!=q (or using std::reverse) also breaks vectorization, but that's less surprising.